A conventional power switching converter is a flyback converter wherein a transformer with a primary winding and a secondary winding is provided for isolating the load from the voltage source. The primary winding is connected to the voltage source through a power switch while the secondary winding is connected to a load by means of a diode and a filtering capacitor is connected in parallel to the load.
When the power switch switches-on, a first current flows on the primary winding and increases from an initial value as a function of the values of the voltage source and of the inductance provided by the primary winding. During this time, no current flows on the secondary winding because the diode is reverse biased and the power is stored in the core of the transformer.
When the switch switches-off, the current on the primary winding is abruptly switched-off and the power that was just stored in the core is transferred into the secondary winding. A second current on the secondary winding abruptly reaches a peak value equal to the peak current reached by the first current multiplied by the ratio between the number of turns of the primary winding and the secondary winding, when the switch is switched-off. The second current starts to decrease as a function of the inductance of the secondary winding and of the voltage across the load.
The amount of power transferred from the primary winding to the secondary winding depends upon the switching duty cycle of the switch. For this purpose, the power switching converter comprises a control circuit for driving the switch, while the control circuit is configured to receive a feedback signal to modify the width of the control pulses of the switch.
The feedback control is provided by means of an optocoupler or an auxiliary winding. In the latter case, the auxiliary winding gives an image of the output voltage, being directly in phase with the secondary winding.
In conditions of light load the power switching converter is typically made to operate in a burst-mode. With this operating mode the converter operates intermittently, with series (bursts) of switching cycles separated by time intervals during which the converter does not switch (idle time). When the load is such that the converter has just entered burst-mode operation, the idle time is short. As the load decreases, the duration of the bursts decreases as well and the idle time increases. In this way, the average switching frequency is considerably reduced and, consequently, the switching losses associated with the parasitic elements in the converter and the conduction losses related to the flow of reactive current in the transformer are reduced. The duration of the bursts and the idle time are determined by the feedback loop so that the output voltage of the converter remains under control.
In the case wherein the feedback of the output voltage is formed by the auxiliary winding, the minimum frequency of the burst-mode operation is determined by the control circuit of the switch. During the burst-mode operation, the control circuit periodically forces the switching-on of the switch with a certain “restart” frequency in order to receive the feedback signal.
Thus, the power switching converter provides a fixed power which is independent from the load. This power is to be dissipated to avoid a situation where under low or zero load the converter goes out of regulation. To this purpose, a dummy load is typically used.
That power to dissipate mainly depends on the “restart” frequency, which should not be chosen too low. In fact, during the time period between two subsequent commutations of the switch, the control circuit is not able to respond to an eventual variation of the load at the output terminal. When a commutation of the switch occurs, the converter responds by providing to the load the required power.
In the worst case, when a variation of the load from zero to a maximum value occurs, the current absorbed by the load is supported by the output capacitor and the voltage drop of the output voltage depends on the value of the capacitance of the output capacitor (the higher the output capacitance, the lower the voltage drop), on the “restart” frequency (the lower the frequency, the higher the voltage drop) and on the maximum output current. A trade-off between burst-mode consumption and the value of the output capacitor is involved during the design phase of the power switching converter.
However, to obtain acceptable power dissipation values a relatively low “restart” frequency is chosen, which means a choice of an excessive output capacitance.
To overcome this drawback, one approach is to use a wake up circuit configured to force the switching-on of the switch when a variation of the load occurs between two subsequent commutations during the burst-mode. This allows low power consumption without using a large output capacitor. However, other issues may arrive. Therefore, additional development in this area is needed.